Treatment of iron or steel in electric furnaces



'March 31. 1925. 1,532,052 A. E. GREENE TREATMENT 0F IRON OR STEEL INELECTRIC FURNACES Filed Jan. 2, 1909 f@ @whim/E,

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i Patented Mar. 31, 1925.

UNITED STATES 1,532,052 PATENT orifice.

ALBERT E. GREENE,

sMELmINe Nn ENGINEERING 'rIoN oE mussolini.

OF CHICAGO', ILLINOIS, ASSIC'NOR TO AMERICAN ELECTRIC COMPANY, 0F ST.LOUIS, MISSOURI, .A CORPORA- TREATMENT 0F IRON O'B. STEEL IN ELECTRICFURNACES.

Application led Januaryv 2, 1909. Serial No. 470,366.

To all whom t may concern:

Be it known that I, ALBERT E. GREENE, a citizen of the United States,residing at Chicago, in the county of Cook and State of Illinois, haveinvented a certain new and useful Improvement in the Treatment of Ironor Steel in Electric Furnaces, of which the following-is a full, clear,concise, and exact description.

My present invention relates to improvements inV44 metallurgicalprocesses and more articularly to the treatment of iron or steel 1nelectric furnaces. The invention has for its object the provision of animproved electric furnace process for treating iron or steel in thecourse of which impurities, such as sulfur, phosphorous and oxygen, areremoved therefrom, principally as a slag, either singly or together, andin such manner as to leave the iron or steel in a purified anddeoxidized condition.

In the making or rening of steel in electric furnaces prior to thisinvention, the usual method has been to make the lining or hearth of thefurnace of basic material, such as burnt magnesite or dolomite, the roofbeing of silica brick. The slag used has been highly basic,containing-say about-15 percent of silica, and requiring a considerableaddition-say 3() percent of iluorspar to act as a flux. The temperatureattainable in the electric furnace is sufficiently high to melt a slagof extreme basicity, and the fumes from the highly basic slagsheretofore generally used have a strong corrosive action on the silicaof the roof, so that the latter requires renewing at frequent intervalsresulting in considerable expense and very serious delays in theoperation of the electric furnace. The fluorspar which becomesvolatilzed has the same corrosive eHect on the roof and this materialfurthermore is quite expensive. The chief reason for using aslag sohighly basic has been to reduce to a minimum the sulfur content of theproduct. I have found, however, that by using a slag containing acomparatively. high percentage of silica, the sulfur contentof the metalmay be reduced to practically the same extent as before and considerableeconomies are eEected in the operating of the furnace and. also animprovement securedl in the quality of the .steel produced.

My invention, accordingly, involves treating iron of steel in a basicelectric furnace y applying thereto a slag comprising'lirne and silicaapproximately in such proportions as to form a neutral mixture, that is,a mixture that is neither strongly acid nor strongly basic, andmaintaining the metal and slag molten under reducing conditions. Theactual percentage of silica in the slag will vary considerably foi"different materials and circumstances, but will generally be between 25and 35 per cent of the total Weight of the slag. Appropriate reducingconditions may be obtained in various ways, as, for example, by the useof suitable carbonaceous reducing agents, such as a gaseous reducingagent acting on the surface of the charge in the furnace. Such reducingconditions, as contemplated by the invention, ensure the reduction ofany iron"l o xide in the slag thereby producing conditions morefavorable for the taking up by the slag of phosphorous .and sulfur, oreither of them.

The essential feature of the improved process of my present invention isthe production in a molten bath of iron or steel of conditions favorable-for the separation therefrom of phosphorous and sulfur, or either ofthem, by combination with slagforming constituents, such as lime andsilica, the phosphorous or sulfur compound thus formed floating on topof the molten bath and being removed in the usual mann er as a slag. Theproduction of conditions favorable for accomplishing these results isattained by maintaining the molten metal and slag at a temperature atwhich the phosphorous or sulfur,`or both, will combine with theslag-forming constituents present, the quantity of silica present beingsuch that the slag is neither strongly acid nor strongly basic, and byestablishing and maintaining suitable reducing conditions.

Thus, the prsent invention involves regulating, by an electric current,the temperature of a bath of molten iron'or steel, covered by a iiuidslag comprising lime and silica, and meanwhile actin upon the bath witha reducing agent, suc as al carbonaceous gaseous reducing agent, forexample. carbon. monoxideico - lUnder such conditions, it appears thatthe lime in the flux is reduced thereby providing calcium which combineswith sulfur to form calcium sulfide, or with phosphorous possiblyforming calcium p iosp ide, respectively. In any event, the combinedaction of the reducing agent andthe fluid limesilica slag producesconditions favorable for the taking up by'the slag of either or bothphosphorous and sulfur.

ln carrying out the inventiomthe reducing action and the presence ofsuitable f luxing agents for forming a fluid lime-silica slag areessential.. I have obtained the desired reducing action by blowing agaseous reducing agent such as carbon monoxide upon the surface of themolten slag covering the molten metal. rllhe necessary reducing actionmay be obtained in various other ways apparent to those skilled in theart. 'lthe carbonof the electrodes in an are furnace contributes towardsthe production of the necessary reducing conditions. Without the inuenceof a reducing actionfacting on lthe bath, the separation of phosphorousor sulfur, as contemplated by theinvention, is not assured orpracticable. lt is, however, important that the reducing' conditions becontrolled. Under. the reducing conditions contemplated by my presentinvention, silica in the slag may bereduced, and the resulting silicon,on account of its high affinity for oxygen, probably materially aids ininsuring reducing conditions. At any rate reduction of silica isinsurance that reducing conditions have been maintained.v

An important feature of the present invention resides in themodification of the heretofore customary highly basic slag by theaddition of a relatively high percentage of silica, thereby giving theslag a considerably lower melting point, rendering it more fluid andfacilitating its refining action. This feature results in a verymaterial saving in electrical energy over what would be required to melta straight lime or highly basic slag. l have found, notwithstanding thepresence of silica in the slag in suihcient quantity to render it fluid,that the eectiveness of such a slag in removing sulfur from the met-alis not substantially lessened an'd the sulfur may be separated from thesteel to practically the same extent as with the heretofore customaryhighly basic slags. By making the slag fluid by means of silica, inaccordance with the present invention, it is possible to pour the slagdirectly from the furnace instead of having to rake it out as may benecessary with the heretofore customary highly basic slags, or insteadof using considerable quantities of fluorspar at an unnecessary expense.

While the process of my invention is not dependent upon the use of anyparticular type of electric furnace, l have in the acresaoa companyingdrawings illustrated two types of furnace suitable for practicing theinvention. fn the drawings- Fig. 1 is a sectional view of an electricfurnace on the line 1-1 of Fig. 2, sho-wing one type of furnace withwhich my invention may be practiced;

Fig. 2 is a sectiona View on the line 2 2 of Fig. l;

Fig. 3 is a vertical section of an induction furnace with which theprocess may be practiced; and

Fig. 4.- is a/plan view of the furnace shown in Fig. 3.

Similar reference characters are used to designate similar partsWherever shown.

Referring to Figs. 1 and 2 the steel casing l of the furnace issupported from trunnions 2 and 3, in order that the furnace may betilted in the usual manner. The side walls 4 are of refractory materialand the Crucible 5 is provided with a basic lining. The furnace 1sprovided with a removable top 6 of silica or other suitable material.

Electrodes extend through the walls of the furnace and dip into the slag8. By tilting the furnace the amount of electrode in the slag may beregulated or it can be kept out of the slag. When the electrodes dip inthe slag a current of low voltage and high amperage is referablyemployed, and the current may e regulated by varying the voltage.lEither direct or alternating current may be employed. One electrode maybe caused to make contact with the meta-l 9 where desired. The trunnion2 is hollow to provide a passage 10 communicating with the tuyre pipe11. Tuyres 12 extend downward through the wall 4 (see Fig. 2). Thereducing gas is introduced under pressure through the passage 10 to thetuyres 12 in any suitable way and at any desirable pressure. The tuyresare inclined 'downward in order that the gas may come into more thoroughcontact witlr the bath. The gas may be blown into the bath or it may beblown into the furnace chamber from above the charge at any suitableplace so as to insure contact of the gas with the slag. lt is to benoted that the gas entering the highly heated chamber will, if colderthan the gas it displaces, spread itself out at the lowest possiblelevel next above the bath by reason of its highei` specific gravity thanthe gas it displaces.

The exhaust gas is preferably let out through the exhaust pipe 13, andsuch gas may be collected on account of its value.

Figs?) and 4 show a well known type of induction furnace. The steelcasing thereof may be supported in any suitable manner. The refractorywalls 15 enclosed in the casing are annular in shape and surround oneleg of the transformer core 16. The core is wound with one or more coils17.

lllO

which serve as the primary of the transformer, the secondary thereofconsisting of the metallic bath 18 contained in the furnace chamber 19.The chamber 19 is provided with a cover, which is preferably made in aplurality of sections 20. Gas is admitted to the furnace chamber 19 fromthe gas main 21 and the flow thereof is controlled by the valve 22.Tuyeres 23, which are connected by means of pipe 24 With'tliey g'asmain, extend through fthe cover 20. The exhaust gasses pass out throughthe exhaust pipe 25.

In practice, I prefer to use an electric furnace of the types described,or of any other suitable type, for the reason that the fluxes maythereby be readily melted and the teinpeinture of the bath may beregulated and controlled. g

Lime is a flux which readily combines with sulfur and under certainconditions with phosphorous. I have found that sulfur may be removedfrom pig iron to less than 0.005 percent by adding lime to a bath of pigiron, maintaining tlie metal and slag in a molten condition at atemperature approximately of 1325o C., and by blowing the bathmaintained at such temperature with a reducing gas containing 23(percent by volume of carbon monoxide (C the remaining content of thegas being nonoxidizing. To maintain the slag fluid at such a lowtemperature, it is necessary that it contain more than 15 percent ofsilica.

To remove phosphorous as a slag at such relatively 10W temperature, itis necessary to'liave a more fluid slag than is required forsulfurvremoval. The slag may contain' as much as 80 percent of silicaand at a temperature of approximately 1400o C., the

. aforementioned gaseous mixture containing about 23 percent by volumeof carbon inon oxide (CO) Will give a satisfactory elimination of both(or either) phosphorous and sulfur from the iron or steel undertreatnient.

Slags in certain cases may contain metallic oxides,such as iron oxide,and vin the case of Bessemer and open-hearth metal this is invariablythe case. After the metal has been treated in the Bessemer converter orYthe open-hearth furnace, it may be further treated in an electricIfurnace by the process of this invention, in which case-the reduc-- ingagent acting upon the molten bath causes the reduction of the ironoxides of the slag, thus recovering the iron and also removingconditions which are detrimental to the removal of sulfur andphosphorous from, the metal. I

As liereinbefore stated, the reducing agent employed in practicing theimproved process of the invention is adapted to act as a reducing agentwith respect. to the flux (or slag) and incidentally with respect to themetallic oxides of the slag. In the practice of the invention I havesecured satisfactory results by theuse of carbonaceous aseous rednfcingagents. vCarbon monoxi e is such a.'

gaseous agent. In practice,I have also foundj.

that the gases from certain blast furnaces, es-

pecially if the gas contains a large amount of carbon monoxide, may beemployed, with satisfactory results, to produce in the slag conditionsfavorable for the removal of phosphorous and sulfur.

For example,

stack furnace gas, containing 22 percent by volume of carbon monoxide00) and 12 percent by volume of carbon dioxide (0Q2) acts in asatisfactory manner as a reducing agent for the flux and accompanyingslag in practicing my present invention. Producer gas, which consistslargely of carbon monoxide, gases containing a. large percentage ofhydrogen' or methane (CIL), and the like, may also be used iii carryingout the invention.

-The process of the invention is of -particular advantage in themanufacture of soft or low carbon steel. The greater or less quantity ofnascent silicon which enters the bath serves to purify it and in a Wayto make up in purifying effect for the lack of carbon in the bath. 6Thesilica in the slag maybe so proportioned that the quantity of nascentsilicon which enters the bath shall be only sufficient to have apurifying effect and Shall not be sufficient to leave any appreciableercentage of silicon in the iinished pro uct. y

I claim:

1. The method of treating iron or steel which consist in applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a basic hearth in an electric furnace, the quantity ofsilica being about 25 to 35 percent. of the total weight of the slag.

2. The method of treating iron or steel which consists in applyingthereto a slag comprising lime and silica and Vmaintaining the metal andslag molten on a basic hearth in an electric furnace, the lime andsilica being approximately in such proportions as to Aform a neutralmixture, that is, a mixture which is neither strongly acid nor stronglybasic.

3. The method of treating iron or steel which consists in applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a .basic hearth in an electric furnace, the percentage ofsilica in the slag being sufficient to practically avoid formationof-'-fumes of lime in the furnace.

4. In the making of soft or lowcarbon steel the method Which'consists inapplying to the bath a slag comprising lime and silica and maintainingthe metal and slag molten on a basic hearth inv an electric furnace, the

. percentage of silica in the slag being so high that silicon is reducedtherefrom and enters 'ift the bath in nascent condition.

5. In the making of soft or low carbon steel the method which consistsin applyin to the bath a slag comprising lime an -silica and maintainingthe metal and Slag molten on a basic hearth in an electric furnace, thepercentage of silica in the slag being so high that silicon is reducedtherefrom and enters the bath in nascent condition in sufficientquantity to remove impurities to a substantial extent but not insnilicieiit' quantity to appear in an appreciable percentage in theinished product.

6. rlhe method of treating iron 6r steel which consists in applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a basic hearth in an electric 'furnace under reducingconditions, the quantity of silica being about 25 to 85 per cent of thetotal Weight of the slag.

7. The method of treating iron or steel which consists iii applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a basic hearth in an electric furnace under reducingconditions, the lime and silica being approximately in such proportionsas to orm a neutral mixture, that is, a mixture which is neitherstrongly acid nor strongly basic.

8. rlhe method of treating iron or steel which consists in applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a basic hearth in an electric furnace While subjected tothe action of a reducing agent, the percentage of silica in the slagbeing suflicient to practically avoid formation of fumes of lime inthe'furnace.

9. The method of treating iron or steel which consists in applyingthereto a slag comprising lime and silica and maintainin the metal andslag molten on a basic hearth in an electric furnace under reducingconditions, the percentage of silica in the slag being so high thatsilicon is reduced therefrom and enters the molten bath in nascentcondition.

10. The method ot treating iron or steel, which consists in applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a basic hearth in an electric furnace While subjected tothe action of a reducing agent, the percentage of silica in the slagbeing so high that silicon is reduced therefrom and enters the moltenbath in nascent condition in sui'iicient quantity to remove impuritiesto a substantial extent but not in sufficient quantity to appear in anappreciable percentage in the finished product.

iin Witness whereof, l, hereunto subscribe my naine this thirt-ieth dayot December, A.. D., 1908.

ALBERT E. GREENE. Witnesses z GEORGE E. lFoLK,

ALFRED H. Moenia. y

